In RFID systems (RFID=Radio-Frequency Identification, i.e., identification with the aid of electromagnetic waves, e.g., in the UHF range (UHF=Ultra High Frequency), or with the aid of magnetic coupling, e.g., in the LF range (LF=Low Frequency) or in the HF range (HF=High Frequency)), there is generally only a limited and low amount of energy available on the side of the transponder. For this reason, in transponder systems operating with electromagnetic coupling (UHF and higher frequencies), the backscatter modulation is used for transmitting the data from the transponder to the reading device (uplink). This is a modulation method wherein the radar cross-section of the transponder is varied depending on the transponder data rate f_Tag. The radar cross-section contains the geometrical dimensions of the transponder as well as the reflection factor indicating which amount of the incident wave (in this case, of the carrier) is radiated back, or reflected. This backscatter modulation results in a spectral characteristic in which the “backscattered” transponder signal is, from a spectral perspective, very close to the carrier signal, see FIG. 5.
In detail, FIG. 5 shows a diagram of a fundamental illustration of the energy of a backscatter signal 10 and carrier signal 12 across the frequency in the classical transmission in a transponder system. In this case, the ordinate describes an amount of an amplitude of the signals and the abscissa describes the time.
As a result of this characteristic, detecting and separating the transponder signal in the receiver module is not trivial since filtering the transponder signal directly from the high-frequency receive signal is not possible because the steep-edge, or narrowband, filters cannot be realized or may only be realized using great effort. A further problem in receiving such backscatter signals is the amplitude ratio of the transponder signal compared to the carrier signal (SCR=Signal-to-Carrier ratio) and the noise (SNR=Signal-to-Noise ratio) which does not allow a direct demodulation of the transponder signal. Amplifying the transponder signal by means of a low signal amplifier (LNA) is not possible since the substantially larger carrier signal would lead to an overdrive of the LNA so that there may be no amplification of the transponder signal in the RF path (RF=Radio Frequency). However, filtering and amplifying is possible in the base band range so that homodyne receiving architectures (direct mix receivers) are used as a standard. The advantage of this approach is that, due to the missing input filtration and pre-amplification, only signals having a good SNR are decodable and, thus, range is given away. In [1], the entire underlying problem is described in detail.
In [1], conventional technology for solving the described central issue is extensively illustrated. Fundamentally, three solutions are described therein.
In current standard reading devices operating in the UHF frequency range, due to the above-described problems, homodyne receiver concepts are used, i.e., the received signal is directly downmixed into the base band. By this, the distance between the transponder signal and the carrier frequency is significantly increased and dimming these unwanted signal portions by filtering is easily realizable.
Further, there are two additional improved solution approaches in the uplink which open up potential for improvement and are found in the literature and in the practical use.
In the first solution approach, a so-called auxiliary carrier is used. Here, an additional signal source with a frequency f_HT is switched depending on the transponder data rate f_Tag whereby the transponder signal is spectrally shifted by ±f_HT. In this case, the transponder signal may be shifted to the extent that it may easily be filtered out of the receive signal in the RF path of the reading device, without the carrier signal having an interfering influence. In this case, the use of an LNA (Low Noise Amplifier) for amplifying the transponder signal would also be conceivable after filtering. The problem with this solution approach is that the additional signal source significantly increases the energy demand of the transponder, resulting in a significantly lower energy range, and the resulting reading range of the transponder may be lower as compared to not using an auxiliary carrier.
The second solution approach with respect to the problem is based on the so-called carrier suppression. In this case, the amplitude of the carrier signal is decreased by hardware or software or a combination of hardware and software. In this approach, the inverse carrier signal is added to the receive signal. The improved SCR (ratio of transponder signal to carrier signal) allows an error-free demodulation of the transponder signal, also enabling the use of an LNA for amplifying in the RF path of the reading device.